The way to keep the Plotter happy was to make available to him accurate groundspeed and drift, Green Satin could do this, provided it was operating normally.

The Janus Array Green Satin front panel

Ground speed and drift were obvious requirements for accurate navigation, especially at low level, and these were derived from the Green Satin doppler radar system.  Green Satin was a Janus array doppler radar with four beams projected from the underside of the aircraft, two angled forward and two angled rearwards, around the centre-line of the aircraft.  See the two diagrams below.

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The diagrams below were all taken from my OCU notes. J Dillon

The Janus Array.

The forward and rear aerial sets will receive a transmitted signal which is reflected back from the ground.  The frequency will be shifted (doppler effect) because of the movement of the aircraft. At this stage in the site I am going to ‘duck out’ of trying to translate the OCU notes to go any deeper into the way in which the different frequencies are ‘beaten’ together, the Discriminators, Velodynes and Integrator motors.

In the diagram above if the aircraft is flying with no drift then the speed with which the front left (red) beam is travelling along track is the same as that of the front right (blue) beam and similarly for the two rearward beams.

In the diagram below there is drift in the system, so now the front red and the rear red beams are going along track faster than the blue beams.  This difference is fed as analogue voltages to a system that will physically align the aerial system into a position where this difference is ‘nulled’, so indicating the drift in degrees turned, and now accurately calculating groundspeed along track.

Diagram of the front panel of the Green Satin doppler, taken from my OCU Notes.  J Dillon

In the diagram above, showing the front panel there are two light indicators on the repeater; a green lamp showed that the unit was functionally OK, producing good output, an orange lamp meant that the system had gone to memory mode. If the system was operating over a smooth sea surface then the radar returns would sometimes not be adequate and the system would freeze with the last recorded groundspeed and drift.  It was important for the Nav Plotter to be very aware of the state of these indicators as Green Satin was an input to the GPI6, and so all position information from the navigation system would be effected by the state of the Green Satin. If it were in memory mode for long then the position information would become progressively more inaccurate depending on the error between true groundspeed and drift, and the last accurate readings frozen in the Green Satin memory.  If the unit remained in memory mode for more than a few minutes then the Plotter would need to use other methods to measure drift, if the aircraft was in a position where radar fixes were possible (over or near land features) then the system could be corrected by inputting frequent radar updates to the GPI6. Any tendency for Green Satin to go to memory could be guaranteed to draw a curse from the Nav Plotter, as it would always mean increased workload for him, and some inaccuracy in his route navigation.  To help with the sensitivity needed between over land and over sea operation there is a land/sea switch bottom right allowing the Plotter to change the selection.